The instant invention relates to the plating or coating of substrates or articles comprised primarily of titanium.
It is known that a coating on a metallic article may serve to reduce galvanic action between the coated article and another article formed of a less noble metal in the presence of an electrolyte as, for example, where titanium fasteners are employed to secure aluminum aircraft structural components. Specifically, galvanic corrosion occurs when a metal such as titanium, which is passive on the galvanic scale, is placed in direct contact with a relatively less noble second metal such as aluminum, which is thus anodic to titanium. The interposition of a third material between the titanium and the other less noble material, as by coating the titanium article with such third material, thus provides galvanic protection therefor by preventing such direct contact therebetween.
Additionally, a coating can further act as an intermediary to reduce sliding friction between a titanium article and another article, such as may be found where a titanium fastener in the form of a bolt or pin is placed in interference fit in the bore of an aluminum structure, or where a self-locking nut is advanced on a titanium bolt. Specifically, titanium exhibits a molecular affinity towards other metals which is likely to cause galling thereof upon sliding contact therebetween. In aerospace applications, such galling of the aluminum structure may critically affect the fatigue performance thereof. A coating on the titanium fastener comprised of a material having a lesser affinity for aluminum than the titanium substrate thereof provides an improved frictional interface between the titanium substrate of the fastener and the aluminum structure by preventing direct contact therebetween. It is noted that such a coating additionally serves to prevent seizing when utilized with titanium-titanium fastener combinations.
Unfortunately, the tendency of titanium to rapidly oxidize when exposed to air, and the deleterious effects of such oxidation on the adherence of a coating applied thereto, greatly complicates any attempt to provide a titanium substrate with such a coating. As a result, the prior art has focused on three approaches for improving the adherence of a coating on a titanium substrate: (i) the use of protective films, conversion coatings, and specialized surface activation techniques prior to application of the coating (ii) the formation of a specific oxide layer having a greater affinity for the metallic coating superposed thereon: or (iii) the formation of a diffusion layer between the titanium substrate and the coating subsequent to the plating thereof.
Examples of the first approach include the interposition of a thin, more noble metal film deposited from ionized solution, as in U.S. Pat. No. 3,164,448 to Pottberg; the formation of a conversion coating by immersion in a solution of fluorosilicate or fluoroborate, as in U.S. Pat. No. 3,725,217 to Hartshorn, Jr.: the pretreatment of a titanium substrate by forming a chromium conversion coating thereon as in U.S. Pat. No. 2,825,682 to Missel et al.: cathodic activation in a non-aqueous acetic-sulfuric-hydrofluoric acid bath, as in U.S. Pat. No. 3,817,844 to Kendall: and the activation of the titanium substrate for subsequent plating by wet peening, pickling with a fluoridic solution, and soaking in a solution containing chromium, fluorine, and arsenic or antimony, as in U.S. Pat. No. 4,340,620 to Mielsch et al.
Unfortunately, such intermediate films and conversion coatings are extremely thin and provide poor adhesion of a second coating superposed thereon, and the surface-activation techniques have not proved wholly effective in preventing the formation of an oxide layer on the titanium substrate prior to the application of the outer coating thereon.
Examples of the second approach include the forming of a non-porous coating upon a titanium substrate comprising higher oxides of titanium by immersing same in an alkaline electrolyte comprising an aqueous solution of sodium fluoride and a hydroxide of sodium or potassium, as in U S. Pat. No. 2,934,480 to Slomin; and the forming of a porous adhesion-promoting oxide coating on a titanium article by anodizing the article in a chromic-hydrofluoric acid bath, as in U.S. Pat. No. 4,473,446 to Locke et al.
Again, such intermediate oxide layers are extremely thin and provide poor adhesion of a second coating superposed thereon. Additionally, an anodic coating of titanium oxide alone, as taught in the '480 patent, fails to supply sufficient lubricity for applications involving the press-fitting of the thus coated titanium article.
An example of the third approach is taught in U.S. Pat. No. 3,691,029 to Raymond, wherein, subsequent to cleaning and acid activation, a first thin electroplated chromium layer is diffusion bonded at 1600-1900.degree. F. to a titanium substrate, whereafter a second, thicker chromate layer is electrodeposited thereupon.
However, the heat treatment necessary to generate a diffusion layer between the coating and the titanium substrate is likely to result in the degradation of the titanium substrate. Additionally, it is noted that where diffusion is avoided, the stripping of the coating from the titanium substrate for the reworking thereof may be accomplished with substantially less work, as by chemical stripping.
It is noted that attempts have also been made to combine two of the aforementioned approaches, as in U.S. Pat. No. 4,236,940 to Manty et al., which teaches the pretreatment of a titanium substrate to form a chromium conversion coating prior to the electroplating thereupon of a relatively thicker chromium layer, whereafter the coated substrate is heat-treated to obtain some degree of diffusion bonding between the electroplated chromium layer and the chromium conversion coating, and between the chromium conversion coating and the titanium substrate.
Ultimately, the aforementioned approaches of the prior art (i) fail to provide sufficient adhesion of the outer coating to the titanium substrate; (ii) fail to provide sufficient galvanic protection: (iii) fail to provide sufficient outer coating lubricity: and (iv) are characterized by costly, and highly specialized method steps not particularly well suited for high volume processing.
It is noted that, due to the failure of the aforementioned approaches, the prior art has resorted to a coating comprising aluminum powder suspended in a plastic binder, such as the one taught in U.S. Pat. No. 4,359,504 to Troy. When properly applied and baked to a predetermined hardness, the coating improves the frictional interface between a titanium article coated therewith and the other metallic article. Additionally, there is no risk of galvanic corrosion between such an aluminum-pigmented coating and an aluminum article placed in contact therewith.
Unfortunately, however, application of an aluminum-pigmented coating is complex and tedious: the coating is applied mechanically through precision spraying nozzles as the article to be coated is rotated and advanced through a spray station, whereafter the coated article is suspended in an oven in order to cure the plastic binder to its final hardened condition. Moreover, while the rate of deposition of the coating on the shank portion of a fastener is relatively controlled, the thickness of the coating on the threaded portion of a fastener is virtually uncontrolled. Indeed, specifications for such coatings typically do not require thickness measurements to be made on the threaded portion of the fastener. As a result, a large variation in prevailing torque occurs when lock nuts are utilized with threaded fasteners having such a coating, in addition to the increased friction problems encountered due to the abrasivenature of the sliding aluminum-aluminum contact where the nuts are also formed from aluminum. Such variation in prevailing torque has been a continuing problem with aerospace fasteners presently in use.